CVE-2026-61626

libass is a portable subtitle renderer for the ASS/SSA (Advanced Substation Alpha/Substation Alpha) subtitle format. From 0.17.4 until 0.17.5, libass event pruning does not guard negative Matroska `ReadOrder` values, allowing an attacker to clear arbitrarily many bits to zero at attacker-controlled positions near the NULL pointer. Exploitation requires a malicious Matroska stream loaded through `ass_process_chunk`, enabled read-order checking, and later pruning of the malicious events. Read-order checking is enabled by default unless `ass_set_check_readorder` is called with zero; pruning must occur explicitly through `ass_prune_events` or through automatic pruning previously enabled with `ass_configure_prune`. On common systems with arithmetic right shift, 32-bit int values, the upper address region reserved to the kernel, and matching binary/kernel architecture, the expected consequence is denial of service. More extensive exploitation on uncommon systems is theoretical and strongly constrained by the target addresses, bit-clearing-only mutation, and ASLR. This issue is fixed in version 0.17.5.

• https://github.com/libass/libass/commit/23da65a130bb8c5b2cf0c7df0dd7d424607f98f1
• https://github.com/libass/libass/releases/tag/0.17.5
• https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm

• https://github.com/libass/libass/commit/23da65a130bb8c5b2cf0c7df0dd7d424607f98f1
• https://github.com/libass/libass/releases/tag/0.17.5
• https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm

• MEDIUMCVE-2017-20283
• HIGHCVE-2017-20286PoC
• MEDIUMCVE-2020-37286PoC
• HIGHCVE-2021-48018PoC
• MEDIUMCVE-2021-48020PoC
• UNSCOREDCVE-2025-58773
• MEDIUMCVE-2025-60494PoC
• UNSCOREDCVE-2026-0461

Related by shared AI tags and CWE weakness class. Browse the full archive.